How Much Sun Does a Greenhouse Need

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Not all greenhouses are created equal. Some are complex and technological. Others have stood for dozens of years without ever relying on electricity. But all greenhouses have one thing in common: They cannot function without sunlight. But just how much sun do they need?

A greenhouse should get at least 6 hours of sun per day. Less than that and it runs the risk of losing its temperature during the night. However, after 10 hours of sun, you start to run the risk of drying out the greenhouse.

This is why partial shade is recommended during the summer.

Greenhouses use glass roofs and walls to let sunlight in, after which the heat created by the sunlight is trapped inside.

The trouble is, you cannot turn the sun on and off. Sometimes you need more than it gives, and sometimes you just need it to chill out before it overheats your plants and dries the water out of their soil.

Plant Sunlight Requirements

As you can imagine different plants, vegetables, herbs, and fruit require completely different sunlight requirements. Just consider the vegetable and fruit selection during different parts of the year, usually what you can during the summer months will differ from winder months. This is because of different sunlight requirements for each crop.

full-sun-partial-shade-vegetables-grow-greenhouse

So how much sun a greenhouse will need largely depends on what you intend to grow. As mentioned, it’s mainly recommended to give a greenhouse 6 to 8 hours of full direct sunlight as this is best for most vegetables and fruits.

There are other vegetables that don’t need as much sun and prefer less direct sunlight.

This table below shows 5 different types of sunlight requirements. These types of sunlight will give you some direction on how much sunlight is needed. Combine this information with the information that comes with the growing suggestions when you buy it and you have all the information needed to grow the perfect crops.

SunlightPlant Type
Full sunlight – 6-8hoursPlants that love a day’s sun exposure. Usually considered 6-8 hours of direct sunlight. Consider afternoon sun will the strongest direct sunlight
Partial sunlight – 4-6 hoursPlants that require between 4-6 hours of sunlight per day. They also benefit from some shade time too, especially the afternoon sunlight. These rays might be too strong for partial sun plants.
Partial shade – 4-6 hoursPartial shade plants, similar to partial sunlight plants, don’t like the strength of the afternoon sun too much. Morning sun is much more suitable. They prefer to be shaded during this time. These plants are not quite as sun-loving as partial sun plants.
Dappled sunDappled sun plants require less sunlight and prefer their sun intake to be filtered as it offers more protection to the plants. Using a Sun Shade Canopy (Amazon link) or filtering the sunlight through other more sun-loving plants would be the best solution.
Full shade – 3-4 hoursDespite the name, sunlight is still needed, only 3-4 hours a day. Try to avoid afternoon sunlight the rays are too strong for full shade plants. Using a Sun Shade Canopy here or placed in shaded areas would be best.
Different types of sunlight to suit all vegetables, flowers, fruits, roots and herbs

Plants grow faster in a greenhouse than outside so taking this information you can now plan where you want your greenhouse and what you want to grow inside to yield the best results. You can statically plan where your plants will be placed inside your greenhouse so they all get the required sun needed. Tools like a Sun Shade Canopy (Amazon link) will help to keep your plants shaded when needed as well as keeping your greenhouse well ventilated and if needed adding extra lighting to increase heat and light exposure for more light-loving plants.

Dealing With the Ball of Fire in the Sky

So how is one supposed to know how to manage the sun? How do you navigate something from which you cannot escape?

To begin with, the sunlight needs of your greenhouse are going to be determined by the specific growing needs of what you plant.

Plants start as seeds, and these seeds will require specific temperatures to “germinate”. Germination is the process of a seed growing into a plant.

But What is Germination?

Germination is a process triggered by temperature. Some plants germinate at different temperatures than others. Take pears as an extreme example. Pear seeds germinate between 32° and 40° Fahrenheit. That puts them right at the edge of freezing. Meanwhile, corn seeds germinate at 75° to 90° Fahrenheit. That’s more than twice as warm!

germination-plants

It would be impossible to grow these plants next to each other without some sort of additional apparatus, such as a UV lamp to warm just the corn, or a fan to cool just the pears.

Strangely, some plants do not even germinate except in extreme temperatures, such as freezing or burning. If you have ever wondered why grass grows back greener after a brush fire, it is because the science of germination accounts for exactly that situation.

In short, your sunlight needs will be determined by what you intend to grow and when you intend to grow it.

Fruits generally have lower heat needs than vegetables; pears are an extreme example, but even strawberry seeds germinate at 40° to 60°. This means that many fruits will require some form of shade built into or around the greenhouse during the summer.

The main issue of the summertime is the longer days. The heat will be manageable at the beginning of the day, but it will only build as more and more sunlight pours into the greenhouse.

What is a Solstice Anyway?

Luckily, nature has provided us a way to navigate that sunlight. Since the Earth rotates as it turns night into day, the sun moves across the sky in an incredibly predictable manner. You may have heard of the summer and winter solstices. These are the dates when the daylight is greatest and least, respectively.

Every day after the winter solstice the sun rises earlier and earlier in the day, then sets later and later. That is, until the summer solstice, after which it begins to rise later and later, and sets earlier and earlier, until the next winter solstice.

This cycle is the essential quality of the seasons on Earth, and it is what farmers have been using to plan their crops for longer than recorded history.

You can plan around this pattern when planting in a green house. Let’s say that you plant in the middle of spring, as the calendar hurdles towards the summer solstice. You know the days are going to get longer. What you can do is cover the roof and walls of the southwest side of your greenhouse with black tarp.

The southwest side is where most of the sunlight is going to enter the greenhouse during the afternoon (at least if you live in the northern hemisphere of the planet). This is because the sun “travels” from the southeast to the southwest.

It is for this reason that the south side of greenhouses is the most at risk of overheating. The plants on the north side will get the least sunlight.

This is a property of how light is diffused by greenhouse glass; everything in a greenhouse is essentially equally distant from the sun. Considering the sun is more than 92 million miles away, your pears being twenty feet north of your corn might not seem relevant.

But when in a greenhouse, the waves of light hitting your plants are not coming directly from the sun. They are being conveyed through the walls and roof of the greenhouse.

In short, the real source of light in a greenhouse is not the sun, but the glass panes just a few feet away from your plants.

For this reason, plants and seeds that require less sunlight and heat should be positioned on the north and northeast portions of the greenhouse, as their correspondent walls will convey less sunlight. This makes these areas a better home for fruits than vegetables or flowers.

No Sunlight? No Problem!

But what if you need more sunlight than nature is giving you? What if you are growing during the winter, or in a city where skyscrapers might block your greenhouse’s view of the sun? Even during the spring and summer, rain can come and obstruct the sun for days at a time.

That is to say nothing of environments further from the equator, where longer winters make planting season into a gloomy second autumn. What do you do when the seasons just don’t provide?

In that case it is best to break down the problem to understand what you need to get out of the sun that the sun is not providing. Are you needing more sunlight? Or is it a combined problem of light and heat?

If your greenhouse is warm enough, but not getting the light it needs, then the solution to your problem is as simple as the right kind of lamp.

Plants enjoy the sun’s rays because of its intensity, but modern light fixtures can compete with this intensity.

How Complicated can Light Be?

See, light exists on a spectrum. Some light hits your eyes more frequently. This is because it has a shorter “wavelength”, and as a result of this shorter wavelength the light takes on a different color. Purple and blue light has the shortest wavelength of all visible light. A short wavelength means the light collides with anything it touches more frequently.

This makes purple and blue LED lights a feast for light-starved greenhouses.

A plant feeding off of a blue light will get nearly double the nutrition than it would from an orange light of the same intensity. It is a simple matter of the blue light “hitting” the plant at a higher frequency than the orange light.

Blue light is recommended over purple light, however. Even though purple light has an even higher frequency than blue light, it can actually be so high that plants can’t absorb it.

There are many kinds of light that are not visible to the human eye for the same reason. In this case, the human eye can identify a color of light (purple) that plants have trouble absorbing.

Okay, That’s Pretty Complicated.

You can use lights to supplement your greenhouse even if it gets direct sunlight.

This is because the big danger of sunlight is the heat more than the light. As stated before, there are many types of light that are not visible, and these types of light are what carry heat with them.

A blue LED light is not going to output the invisible ultraviolet or gamma rays the sun does. This makes them safe to use to your heart’s content. Just so long as you do not drive up your electrical bill.

Giving your greenhouse enough sunlight is all a matter of knowing what you are growing.

Specifically, you must know how much heat it needs to germinate. Then you just need to keep the sunlight from drying up its water supply.

To manage the heat, you need to manage the sunlight. And if the sun becomes too much for the plants to bear, there is always the option to use LED lights to keep your plants going. And as long as you keep your plants happy with ample, digestible light, they’ll keep you happy as well.

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